For the files/folder panel in details view I have created a separate column to show the extension.

That's working fine for files, but for folders it can present strange results.
When a folder name contains one or more dots (full-stops), all text following the last dot will be handled as if it's a file extension and will populate the 'Ext.' column, rendering the column very messy.

Is there some fix so that for folders no (assumed) extension is displayed?

Can't confirm this, not even for the TC 9.20 beta versions. The name of directories is always shown in full, the extension not separated from the rest of the name, even if showing the option to show the extension in the extension column is enabled (which I have always enabled anyway).

Please try running TC with a clean wincmd.ini, like this

Code:

totalcmd.exe /i="%temp%\fresh.ini"

(replace totalcmd.exe with totalcmd64.exe in case you're using TC x64.) Create the custom columns view (obviously) and try again. If TC isn't separating the extension for directories there, it might be some TC option that causes this (which might indicate a bug, but that remains to be seen).

Could you confirm you get different results with such settings?
If so, I will consider resetting ini's and that sort of stuff.
(I'd rather not since I've got pretty much everything working very nice and solid by now)

Aha, you've added tc.ext in a column. Then it's obvious why you're seeing what you're seeing. Neither of my custom columns views use this field because I don't use it (because IMO it doesn't make any sense, even less in a separate column).

Technically TC is showing correct results since directories are essentially files with a special (directory) flag. On the other hand, the user might argue that directories don't have extensions. There's no simple solution to this, and in regards to custom columns, it would require to add another field to the internal tc plugin so that everyone can be satisfied.